1. Field of the Invention
The present invention relates to a coil spring modeling apparatus capable of producing a reactive force (repulsive force) corresponding to compression of a helical spring such as a suspension coil spring, and a method of controlling the same.
2. Description of the Related Art
As an example of a vehicle suspension system, a McPherson-strut-type suspension is known. The McPherson-strut-type suspension comprises a coil spring, and a strut (a shock absorber) provided inside of the coil spring. The coil spring is compressed by a load applied from above the coil spring, and is extended and retracted in accordance with the load. The strut is also extended and retracted.
In the McPherson-strut-type suspension, in order to reduce the sliding resistance of a strut, offsetting a force line position (FLP) of a coil spring from the center line of the coil spring is known. For example, the force line position (FLP) of a coil spring is set at a position where the friction of the strut is minimal. For this reason, the relationship between a force line position (FLP) of a coil spring and the sliding resistance of a strut must be specified. However, producing a variety of coil springs whose force line positions are different by way of trial is time consuming and costly. Thus, instead of using the coil spring, using a coil spring modeling apparatus has been proposed.
For example, a coil spring modeling apparatus disclosed in, U.S. Pat. No. 7,606,690 (Document 1) is known. Also, an improved coil spring modeling apparatus is disclosed in “Research of Effect of Coil Spring Reaction Force Line on Vehicle Characteristics by Universal Spring” (Document 2), on pages 21 to 24 of the proceedings, presentation of which was made in the conference held by the Japan Society of Spring Engineers (in Nagoya) on Nov. 1, 2013, and “Experimental Study on the Effect of Coil Spring Reaction Force Vector on Suspension Characteristics” of SAE 2014 (Document 3), presentation of which was made in the U.S. (Detroit) on Apr. 8, 2014. The coil spring modeling apparatus disclosed in the above documents has a Stewart-platform-type parallel mechanism comprising six hydraulic cylinders. By actuating each of the hydraulic cylinders by fluid pressure, a reactive force corresponding to compression of a coil spring can be produced.
In the McPherson-strut-type suspension, when a coil spring is compressed between the lower spring seat and the upper spring seat, it is known that torsion (a relative change of rotational position) is produced between the lower end turn portion and the upper end turn portion in accordance with the amount of compression. While an upper bearing is disposed between the upper spring seat and a mount portion on the side of a vehicle, the upper bearing has some degree of friction (rotational resistance). Therefore, if the coil spring is compressed, by the friction of the upper bearing, a moment in the rotational direction is produced between the lower end turn portion and the upper end turn portion. This moment produces a kingpin moment (KPM; a moment about a kingpin axis). The kingpin moment (KPM) becomes a factor which adversely affects the steering performance of a vehicle. The kingpin moment (KPM) changes in accordance with a geometric positional relationship between the kingpin axis and the strut axis. Also, the kingpin moment (KPM) may sometimes be affected by a force line position (FLP).
The coil spring modeling apparatus conceived by the inventors of the present invention includes an actuator unit (for example, a Stewart-platform-type parallel mechanism) which is actuated by fluid pressure. A strut set in the coil spring modeling apparatus includes a first strut element (for example, an outer tube) and a second strut element (for example, a rod). An upper end of the coil spring modeling apparatus is supported by a base member just like the actual mount portion of a vehicle. A bearing is disposed between the base member and the upper spring seat.
In measuring a kingpin moment (KPM), a torque about the kingpin axis is applied to the first strut element by, for example, a push-pull testing unit. The torque is conveyed to the upper spring seat via the actuator unit. Accordingly, if the friction of the bearing is zero, the upper spring seat rotates as much as the lower spring seat. However, in reality, since the bearing has friction, rotation of the upper spring seat meets resistance. Consequently, the actuator unit is twisted and the force line position (FLP) is also changed, which causes a problem that the kingpin moment (KPM) cannot be measured accurately.